The present invention relates to a method of forming a silicon nitride film used for, e.g., the capacitor of a semiconductor integrated circuit.
As a dielectric film forming the capacitor of a DRAM (Dynamic Random Access Memory) constituted by one transistor and one capacitor, a silicon nitride film is used in terms of high availability and easy handling of a film formation material.
Formation of the silicon nitride film adopts CVD (Chemical Vapor Deposition) using, as source gases, dichlorosilane (SiCl2H2) as silicon chloride gas, and ammonia gas. If the dichlorosilane supply amount increases in forming a silicon nitride film by CVD, particles as a by-product mainly containing ammonium chloride are undesirably produced in the film formation atmosphere. To prevent this, the dichlorosilane supply amount is set about ⅕ the ammonia supply amount.
In general, a dielectric film forming a capacitor must have a critical voltage at 10 nA/cm2 of 1 V or more due to the following reason. That is, the operation voltage of a semiconductor integrated circuit using a DRAM is about 1.8 V at present. The critical voltage of the capacitor must be about half this voltage, and must be substantially 1 V or more with a margin. If the operation voltage of the DRAM decreases in the near future, the critical voltage of the capacitor may suffice to be 0.7 V or more.
The dielectric film of the capacitor requires a dielectric characteristic of 4 nm or less in film thickness in conversion into silicon oxide. Hence, the film thickness of silicon nitride is decreased to about 5 nm.
In summary, the silicon nitride film used as a dielectric film forming the capacitor of a DRAM must attain a critical voltage of 0.7 V to 1 V or more with a small film thickness of 5 nm.
However, according to the conventional silicon nitride film formation method, hydrogen readily enters a formed silicon nitride film. When a thin film is formed as described above, a leakage current is readily generated, failing to obtain a critical voltage of 0.7 V or more.
The present invention has been made to overcome the conventional drawbacks, and has as its object to form a silicon nitride film with a film thickness of about 5 nm and a critical voltage of 0.7 V or more while suppressing generation of any leakage current.
To achieve the above object, according to the present invention, there is provided a silicon nitride film formation method comprising the steps of heating a substrate to be subjected to film formation, and supplying silicon tetrachloride gas and ammonia gas to the substrate heated to a predetermined temperature, wherein the ratio of a partial pressure of the silicon tetrachloride gas to a partial pressure of the ammonia gas is set to not less than 0.5.